Devices for converting electromagnetic energy into electrical energy have been in existence for some decades now. These devices are often known as photovoltaic devices. A fundamental property of any such device is its efficiency, that is, the amount of electrical energy extracted when compared to the electromagnetic energy received. An aim of photovoltaic cell design is to achieve the highest possible efficiency, that is, to extract as much electrical energy as possible from a given amount of received electromagnetic energy.
Perhaps the most common form of photovoltaic device is the solar cell, designed to harness the energy of the sun for electrical power generation. Solar cells are found across a broad range of applications, from pocket calculators to solar powered aircraft. One design consideration when attempting to maximise the efficiency of solar cells is the range of wavelengths at which light from the sun is transmitted.
There is also interest in another form of photovoltaic device, for power transmission. Conventionally, electrical power is carried from its source to the point at which it is to be used by electrical wiring. However, by using photovoltaic devices it is possible to receive energy that has been transmitted in the form of high frequency electromagnetic radiation.
Efforts have been made to design photovoltaic devices for receiving energy from a narrow spectral linewidth light source, typically a laser, within a power transmission system. For example, an electrically powered laser may generate electromagnetic radiation which may in turn be received by a photovoltaic device able to convert it back into electrical energy. Photovoltaic devices are often referred to as Laser Power Converters (LPCs) in this context.
Broadly speaking, power transmission using photovoltaic devices lends itself to situations in which conventional electrical wiring either suffers from unwanted electromagnetic interference or is difficult to install. For example, electrical wiring faces interference when located in close proximity to high voltages. This interference effect is negligible if energy is carried in the form of electromagnetic radiation through fibre optic cables. The use of fibre optic cables has the additional advantage of reducing the possibility of unwanted sparking, which can be dangerous in a variety of circumstances.
The application of photovoltaic devices for power transmission in circumstances where it is difficult or impossible to install cabling of any kind may allow electromagnetic energy to be transmitted through free space For example, satellites orbiting the earth are in an ideal position to generate electrical energy using solar power. It may be possible for this energy to be beamed down to earth by laser where it can be subsequently reconverted to electrical energy by photovoltaic cells at a receiving station. This may allow energy to be transmitted to regions of the earth which cannot for practical reasons be connected to an existing power supply.
In addition, LPCs may also be useful for applications including remote powering of subcutaneous electrical devices for medical diagnostics and related applications, and line-of-sight wireless powering of electronic equipment.
Electromagnetic radiation generated by a laser is substantially monochromatic, i.e. a laser emits over a very narrow range of wavelengths, in contrast with the broad range of wavelengths transmitted from the sun. Therefore the design considerations for maximising the efficiency of LPC devices differ from those for maximising the efficiency of solar cells.
For example, V. Andreev et. al. “High current density GaAs and GaSb photovoltaic cells for laser power beaming”, Photovoltaic Energy Conversion Conference, page 761 (2003) reports efficiencies of above 50% being achieved for GaAs-based photovoltaic devices at a wavelength in the region of 820 nm to 850 nm. This document describes photovoltaic devices formed on a GaAs substrate which include a Distributed Bragg Reflector formed of alternating layers of AlAs and GaAs in order to increase the responsivity of the device at wavelengths between 830 nm and 870 nm. The document further describes the use of iodine lasers operating at a wavelength of 1315 nm in power transmission, and investigates the possibility of photovoltaic devices based on a GaSb substrate for receiving electromagnetic radiation at this wavelength.
A. W. Bett et. al. “III-V solar cells under monochromatic illumination”, Photovoltaic Specialists Conference, page 1 (2008) provides a comparison of the efficiency achieved using photovoltaic devices having active regions formed of different materials, namely GaAs, GaInP and GaSb. A peak efficiency of 54% was achieved using a GaAs active region, the peak being achieved at a wavelength of around 810 nm. The document also describes a dual junction GaAs photovoltaic device employing a Distributed Bragg Reflector.
Although the above-described efforts to design LPCs for particular wavelengths have met with some success, they are limited in their application. In particular, although a maximum efficiency of above 50% has been achieved, this is achieved at wavelengths of around 810 nm to 870 nm. In practice, such wavelengths are of limited use. For example, at this wavelength, electromagnetic radiation is not “eye-safe” and skin-safe. “Eye-safe” radiation occurs at wavelengths of around 1.4 μm and above, and provides substantially lower risk of damaging the human eye than radiation at shorter wavelengths. Eye-safe radiation is generally considered to be skin-safe. There is a clear benefit in using eye-safe radiation when using lasers to transmit power.
Another disadvantage of electromagnetic radiation having a wavelength in the region of around 810 nm to 870 nm is the attenuation of such radiation that occurs as it passes through either conventional optical fibres or the atmosphere. In either case, energy is lost before it is received by the photovoltaic device.
For these reasons, investigations have been made into power transmission using electromagnetic radiation at longer wavelengths. However, the efficiencies achieved at longer wavelengths have been much lower than those in the 800 nm region.
S. J. Wojtczuk et. al. “Long-wavelength laser power converters for optical fibers”, Photovoltaic Specialists Conference, page 971 (1997) discusses photovoltaic devices for receiving electromagnetic radiation at wavelengths of 1.55 μm and 2.1 μm. In this context, the document describes photovoltaic devices using InGaAs formed on an InP wafer. The maximum reported efficiency for photovoltaic cells using InGaAs is around 35% at a wavelength of 1.55 μm.
H. Miyakawa et. al. “Photovoltaic cell characteristics for high-intensity laser light” Solar Energy Materials & Solar Cells, p 253 (2005) also describes InGaAs/InP photovoltaic devices. However, the peak efficiency described in this document is around 24% at a wavelength of 1480 nm.
It can be seen that the efficiencies achieved in these two systems are fairly low and nowhere near even 50%. Devices with such efficiencies are of limited benefit in a practical LPC system.